with an uncertainty of 3 arcmin (radius, 90% containment, including
systematic uncertainty). The BAT light curve showed a single-peaked
structure with a duration of about 0.2 sec. The peak count rate
was ~7000 counts/sec (15-350 keV), at ~0 sec after the trigger.
The XRT began observing the field at 14:44:16.3 UT, 74.0 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source with an enhanced position: RA, Dec 234.9351, 64.5401 which
is equivalent to:

RA(J2000) = 15h 39m 44.43s
Dec(J2000) = +64d 32' 24.2"

with an uncertainty of 3.3 arcseconds (radius, 90% containment). This
location is 223 arcseconds from the BAT onboard position. This position
may be improved as more data are received; the latest position is
available at http://www.swift.ac.uk/sper. We cannot determine whether
the source is fading at the present time. This position is more than
2-sigma away from the BAT position, and so possibly unrelated to the
trigger.
A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density consistent with the Galactic value of 2.04
x 10^20 cm^-2 (Willingale et al. 2013).
UVOT took a finding chart exposure of 250 seconds with the U filter
starting 78 seconds after the BAT trigger. No credible afterglow candidate has
been found in the initial data products. The 2.7'x2.7' sub-image covers none of
the XRT error circle. The 8'x8' region for the list of sources generated
on-board covers 100% of the XRT error circle. The list of sources is typically
complete to about 18 mag.
Burst Advocate for this burst is D. Kocevski (dankocevski AT gmail.com).
Please contact the BA by email if you require additional information
regarding Swift followup of this burst. In extremely urgent cases, after
trying the Burst Advocate, you can contact the Swift PI by phone (see
Swift TOO web site for information: http://www.swift.psu.edu/too.html.)

GCN Circular #19480
D. Kuroda, K. Yanagisawa, Y. Shimizu, H. Toda (OAO, NAOJ),
S. Nagayama (NAOJ), M. Yoshida (Hiroshima), K. Ohta (Kyoto)
and N. Kawai(Tokyo Tech)
report on behalf of MITSuME and OISTER collaboration:
We observed the field of GRB 160601A (Kocevski et al., GCNC 19478)
with the optical three color (g', Rc and Ic) CCD camera attached
to the MITSuME 50cm telescope of Okayama Astrophysical Observatory.
The observation started on 2016-06-01 14:43:48 UT (~46 sec after
the burst). We did not find any new point source within the XRT
circle (Kocevski et al., GCNC 19478) in all the three bands.
Three sigma upper limits of the OT are listed below.
We used SDSS-DR8 catalog for flux calibration.

GCN Circular #19482
K. Yanagisawa, D. Kuroda, Y. Shimizu, H. Izumiura(OAO/NAOJ),
M. Yoshida (Hiroshima), K. Ohta(Kyoto) and N. Kawai(Tokyo Tech.)
report on behalf of the MITSuME collaboration:
We observed the field of GRB 160601A (Kocevski et al., GCN 19478)
in Ks-band with a wide-field near infrared imager at Okayama
Astrophysical Observatory (Japan). The imager has effective
aperture of 0.91 m.
Observations started from 14:57 UT on 1st June, 14 min after
the BAT trigger, to 15:22 UT. The total exposure of 10.0 min
was successfully obtained.
In our co-add image, we did not find any new point source within
the XRT error circle down to the limiting magnitude of
Ks=16.1 (Vega, S/N=3). The photometric calibration was made
against 2MASS field stars.

GCN Circular #19483
E. Mazaeva (IKI), E. Klunko (ISTP), A. Volnova (IKI), I. Korobtsev
(ISTP), A. Pozanenko (IKI) report on behalf of larger GRB follow-up
collaboration:
We observed the field of the Swift GRB 160601A (Kocevski et al., GCN
19478) with AZT-33IK telescope of Sayan observatory (Mondy) starting on
June, 01 (UT) 15:00:41. We obtained several images in R-filter under
good weather conditions and FWHM of 1.5 arcsec. Using only part of
images available we marginally detect (S/N ~2) a source at the edge of
Swift-XRT position (Kocevski et al., GCN 19478). Coordinates of the
source are (J2000) 15:39:44.43 +64:32:19.3 with uncertainty of 1 arcsec.
Preliminary photometry of the source is R~22.3, and details of
observations are following

with an uncertainty of 3.3 arcsec (radius, 90% confidence).
This position may be improved as more data are received. The latest
position can be viewed at http://www.swift.ac.uk/xrt_positions. Position
enhancement is described by Goad et al. (2007, A&A, 476, 1401) and Evans
et al. (2009, MNRAS, 397, 1177).
This circular was automatically generated, and is an official product of the
Swift-XRT team.

Visual inspection of the SDSS at the above locations shows a faint, marginally detected object coincident with source (2), but no object at the position of source (1). We thus consider source (1) to be a potential afterglow candidate, though further observations are needed to test its variability.

GCN Circular #19486
A. Maselli (INAF-IASFPA), B. Sbarufatti (INAF-OAB/PSU), D.N. Burrows
(PSU), T.G.R. Roegiers (PSU), P.A. Evans (U. Leicester), S.L. Gibson
(U. Leicester), J.P. Osborne (U. Leicester), A. Melandri (INAF-OAB), P.
D'Avanzo (INAF-OAB) and D. Kocevski report on behalf of the Swift-XRT
team:
We have analysed 7.9 ks of XRT data for GRB 160601A (Kocevski et al.
GCN Circ. 19478), from 94 s to 23.6 ks after the BAT trigger. The data
are entirely in Photon Counting (PC) mode. The enhanced XRT position
for this burst was given by Evans et al. (GCN Circ. 19484).
The light curve can be modelled with a power-law decay with a decay
index of alpha=0.49 (+0.14, -0.15).
If the light curve continues to decay with a power-law decay index of
0.49, the count rate at T+24 hours will be 2.0 x 10^-3 count s^-1
The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00688452.
This circular is an official product of the Swift-XRT team.

GCN Circular #19488
S. Schulze (PUC, MAS), D. Malesani (DARK/NBI), A. A. Djupvik (NOT) and P. Jakobsson (U Iceland) report on behalf a larger collaboration:
We re-observed the field of the short GRB 160601A (Kocevski et al., GCN 19478) with the Nordic Optical Telescope (NOT) equipped with the AlFOSC imager. Our first 600-s exposure in the r band started on June 1 at 21:14:48 UT (13.5 hr after the GRB).
We cleanly detect the two objects reported in Malesani et al. (GCN 19485).
Object (1) (RA, Dec. = 15:39:43.97, +64:32:30.5) faded from 22.85 +- 0.10 to 23.36 +- 0.10 mag. The fading corresponds to a decay slope of 0.64 +- 0.25.
Object (2) (RA, Dec. = 15:39:44.91, +64:32:32.1) exhibits no variability beyond the 1.7 sigma confidence level.
These properties let us conclude that object (1) is the afterglow of the short GRB.

GCN Circular #19489
T. Nakaoka, Hirochi, J., Yoshida, M. and Kawabata, K. S. (Hiroshima
Univ.) report on behalf of the OISTER collaboration
We performed I and Ks bands imaging observations around the error
circle of GRB 160601A (Kocevski et al., GCNC 19478) with the optical
infrared simultaneous camera HONIR attached to Kanata telescope of
Hiroshima University. We started the observations at 2016-06-01 16:42
UT. We did not find any optical-infrared counterpart of the burst. Five
sigma upper limits (Vega magnitude) are listed below.

GCN Circular #19491
J. C. Tello, Y. Hu (IAA-CSIC), A.J. Castro-Tirado (IAA-CSIC, ISA-UMA), S.
Oates (Warwick U.) on behalf of a larger collaboration, report:
We observed the field of GRB 160601A (Kocevski et al., GCN 19478) with the
Gran Telescopio Canarias (GTC) with the OSIRIS instrument and acquired
6x100s images starting 01:37:31 UT and ending 01:49:29 UT on July 2nd
(10h54m-11h06m after the Swift trigger) and we detect the afterglow
candidates reported by Malesani et al. (GCN 19485).
We also find like Schulze (GCN 19488) that Object 1 is fading and at the
time of our observations was at a magntude of ~ 23.5 in the Sloan_r filter
and calibrated to SDSS Release 6.
We note that our observations precede Schulze's by 2.5 hours and are
slightly fainter but not much more than the uncertainty of these
preliminary results, implying either a rebrightening event or a plateau due
to either afterglow behaviour or host galaxy.

GCN Circular #19492
S. B. Cenko (NASA/GSFC), W.-F. Fong (Arizona), A. J. Levan (Warwick), A. Cucchiara (STScI), E. Berger (Harvard), and D. A Perley (DARK) report on behalf of a larger collaboration:
We imaged the location of GRB160601A (Kocevski et al., GCN 19478) with the Gemini Multi-Object Spectrograph (GMOS) mounted on the 8m Gemini North telescope. We obtained a series of 5 x 180 s r-band images beginning at 6:02 UT on 2016 June 2 (~ 15.3 hr after the BAT trigger). The candidate optical afterglow proposed by Malesani et al (GCN 19485; their Source 1) is confirmed to be fading (see also Schulze et al., GCN 19488, Tello et al., GCN 19491). Using nearby point sources from SDSS for calibration, we measure a magnitude of r = 23.69 +/- 0.05 at this time.
We note that the source appears to be (marginally) extended in our GMOS images, and therefore host contamination may to be present in the above magnitude estimate.

with an uncertainty of 1.9 arcmin, (radius, sys+stat,
90% containment). The partial coding was 66%.
The mask-weighted light curve shows a single pluse
starting ~T-0.1 sec, and ending around ~T+0.15 sec.
T90 (15-350 keV) is 0.12 +- 0.02 sec (estimated error
including systematics).
The time-averaged spectrum from T-0.02 to T+0.12 sec is
best fit by a simple power-law model. The power law
index of the time-averaged spectrum is 1.04 +- 0.24.
The fluence in the 15-150 keV band is 6.6 +- 1.1 x 10^-08
erg/cm2. The 1-sec peak photon flux measured from T-0.45
sec in the 15-150 keV band is 0.9 +- 0.2 ph/cm2/sec. All
the quoted errors are at the 90% confidence level.
The results of the batgrbproduct analysis are available at
http://gcn.gsfc.nasa.gov/notices_s/688452/BA/

GCN Circular #19495
Y. Saito, Y. Tachibana, T. Yoshii, T. Fujiwara, Y. Ono, Y. Muraki, S. Harita,
T, Ozawa, K. Saisho, Y. Yatsu, and N. Kawai (Tokyo Tech)
report on behalf of the MITSuME collaboration:
We searched for the optical counterpart of GRB 160601A (D. Kocevski et al., GCN Circular #19478)
with theoptical three color (g', Rc, and Ic) CCD cameras attached to the
MITSuME 50 cm telescope of Akeno Observatory, Yamanashi, Japan.
The observation started on 2016-06-01 23:43:35 UT (33 sec after the burst).
We did not find any new point source within XRT circle in all three bands.
We obtained following limits for the magnitudes.

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.02 in the direction of the burst
(Schlegel et al. 1998).

GCN Circular #19497
S. B. Cenko and E. Troja (NASA/GSFC) report on behalf of a larger collaboration:
We imaged the location of the optical afterglow (Malesani et al., GCN 19485) of the short GRB 160601A (Kocevski et al., GCN 19478) with the Large Monolithic Imager on the 4.3-m Discovery Channel Telescope in Happy Jack, AZ. We obtained a series of 180 s exposures in the r-band filter beginning at 5:00 UT on 2016 June 3 (~ 1.6 d after the Swift trigger). We do not detect any significant emission at the location of the optical afterglow to a limiting magnitude of r > 25.0 (calculated with respect to nearby point sources from SDSS). Compared to the most recent detection with Gemini (Cenko et al, GCN 19492), our limits imply a significant steepening of the afterglow decay, with a lower limit on the power-law index of alpha > 1.3 (compared to alpha ~ 0.6 at earlier times; Schulze et al. 19488).
We thank the observers (T. Farnham, M. Knight, and L. Feagan) and the DCT staff for their assistance with these observations.